powerZONE Products for the week of May 12, 2003
Siliconix Says . . .
Si9122/3/4: 500-kHz Controllers With Two Integrated
MOSFET Drivers
Provide up to 92% Efficiency in Dc-to-Dc Conversion Applications
The industry's first 500-kHz PWM controller ICs to combine two integrated
MOSFET drivers and integrated signal drivers for secondary synchronous rectification
were announced today by Siliconix Incorporated, an 80.4%-owned subsidiary
of Vishay Intertechnology, Inc.
Designed to provide a compact and efficient solution for fixed telecom applications requiring power supplies in the range of 50 W to 150 W, the new half-bridge (Si9122, Si9123) and push/pull (Si9124) devices operate over a 12- to 72- input voltage range, drive both primary-side MOSFETs, and provide timing for the secondary synchronous rectifiers.
Each device incorporates a secondary-side FET drive signal for synchronous rectification and a direct primary MOS drive (1-A maximum) on the high and low side. A high-voltage start-up circuit is also integrated. Use of low-on-resistance MOSFETs as secondary synchronous rectifiers provides a significant increase in the converter efficiency - as high as 92%, even for low output voltages - when compared to using Schottky diodes.
The new ICs are compliant with the ETSI 300 132-2 standard, handling input voltages up to 100 V for 100 ms. Voltage-mode control with voltage feed-forward compensation provides for excellent line transient response. Programmable features include an input voltage detector, soft-start, a current-limit, and a programmable oscillator frequency from 200 to 600 kHz. All three devices also offer short-circuit protection, either through frequency foldback (Si9122) or hiccup mode (Si9123, Si9124), in addition to undervoltage lockout and thermal shutdown.
On-chip control of the necessary dead-time delays between the primary
and secondary synchronous signals maintains high efficiency. An external
resistor sets the switching frequency from 200 kHz to 600 kHz.
analogZONE Says...
The telecom and networking market isn't exactly in a growth mode at the moment, but that just gives design engineers more breathing room to work on the next generation of equipment, and in the constraints of real estate the 48-V market in particular is looking for higher and higher power supply efficiency.
The novelty here - and it is a novelty that will be picked up by design engineers - is the provision of a secondary side control to the equation.
The input dc voltage is switched by driver-controlled external MOSFETs (high/low side for the 9122 and 9123, push-pull for the 9124) at a switching rate of between 200 kHz and 600 kHz (set by a single external resistor.) The switching voltage is transformed to a secondary where MOSFETs, instead of Schottkys, are used to rectify the voltage to dc. However, the MOSFETS are controlled by secondary drivers on the IC coupled to them by a pulse transformer.
Controlling the switching times on both sides increases the overall efficiency to over 90%. Yes, there are added costs in the circuit particularly MOSFETs instead of diodes and the addition of the pulse transformer (with driver logic as well for the 9123 and 9124) but those costs will be quickly paid back with the efficiency. A standard comparator (for error voltage) and an opto coupler feed back to the IC for voltage control and current sense. The 9122 uses frequency foldback for current overload control, while the other two use a "hiccup" mode. The latter two also have a soft-start control.
Nominal operating voltage is 12 V with quiescent depending on load, but up to 15 mA. Shutdown is a typical 0.1 mA.
Nice products that fall into a "green" category very well. We hope we can see Siliconix close the input loop in the near future by making it possible to operate the system from universal line voltage through a simple rectifier/filtering. To do that the input would need to tolerate up to 260 V at start-up instead of the 100 V here.
These products will sell extremely well and are priced very attractively.
The Si922/23/24 are in production now with the Si922 in TSSOP-20 or SO-20 and the Si9123/24 in TSSOP-16. All are priced at $1.95 in 10-k piece lots.
Data Sheet (Si9122)
Data Sheet (Si9123)
Data Sheet (Si9124)